Membranes act as selective barriers allowing preferred passage of certain components and hindering passage of other components through various mechanisms: differences in solubility, diffusion, differences in electric charge, polarity, size and shape. The usefulness of a membrane can be characterized by the following main properties: degree of selectivity provided for a desired separation, permeability considerations, mechanical stability (creep and compaction considerations), chemical stability (hydrolytic stability, allowable pH range, microbial resistance, oxidative resistance, etc.), fouling resistance and temperature stability.
Membranes have proven to be reliable and affordable devices for decontaminating water. The cost of these membranes, however, is still higher than most societies that suffer from inadequate water sources can afford. Advances in materials may help to reduce membrane costs.
There are currently four commonly accepted classes of membrane based on the size of the material they will remove. Moving from the smallest to largest pore size, these are Reverse Osmosis (RO), Nanofiltration (NF), Ultrafiltration (UF), and Microfiltration (MF) membranes.
The reverse-osmosis (RO) membrane technique is considered the most promising for brackish water and seawater desalination. It is also used for treatment of slightly polluted water. It uses dynamic pressure to overcome the osmotic pressure of the salt solution, hence causing water-selective permeation from the saline side of a membrane to the freshwater side. Salts are rejected by the membrane, and separation is accomplished. The RO membrane is nonporous. Water dissolves in the membrane's skin, the active layer that determines the membrane's properties, and then permeates the membrane by diffusion. The skin is of the order of 30-200 nm thick and is designed to reduce the hydraulic resistance to water passage.
The membrane main properties, like flux and rejection, are controlled by the skin thickness and integrity. Current RO membranes are sensitive to high and low pH, solvents, oxidizing materials, etc. Better membranes are needed to increase the flux, with high chemical and mechanical stability that may prevent fouling with longer work life. Improved RO membranes are the main key for cost reduction in desalination processes. High flux membranes may reduce the energy needs and the main investments per unit product of fresh water produced.
Nanofiltration (NF) is a membrane liquid separation technology that is situated between reverse osmosis (RO) and ultrafiltration (UF). The process of reverse osmosis normally removes solute molecules dissolved in the solution, in the range of 0.0001 micron in diameter and smaller, whereas the process of nanofiltration removes larger in size molecules in the 0.001 micron range. The nanofiltration technology started three decades ago in order to perform the membranal RO process at a low pressure with a practical flux of water, due to the high pressures that usually are used in the RO process ensuing in a considerable energy cost. Those “low-pressure high flux reverse osmosis membranes” became to be known as nanofiltration (NF) membranes. The first applications of the NF process were reported in 1987-1989. From the beginning, the water industry (especially for drinking) has been the major application area for nanofiltration. The chronological reason for this is that NF membranes were basically developed for reducing the concentration of the inorganic salts of low solubility (softening), and the NF membranes are still sometimes denoted as “softening” membranes.
Nanofiltration membranes are used to partially remove heavy salts and large organic molecules from water for treating slightly polluted surface water and as a pretreatment for desalination processes. Nanofiltration membranes contain pores with diameters in the range of 3 nm. An electrical charge applied to the nanofilter influences salt rejection. Water passage through the membrane is aided by capillary motion through the pores.
Ultrafiltration (UF) membranes have pore diameters in the range of 10-100 nm. Microfiltration (MF) membranes have larger pore diameters, up to 1 μm. Separation of contaminants from water using these membranes is based on simple filtration, which depends upon the size of the contaminant particles in solution and the size of the pores. The membranes retain large molecules, mainly organic molecules and suspended matter. The UF membrane is the modern solution for removing bacteria and viruses from water. MF membranes are used for the removal of suspended particles, and in some cases may also provide protection against bacteria and most viruses. UF and MF are used in combination extensively in wastewater treatment equipment, such as membrane bioreactors, or to clean treated water and surface water.
Water is considered to be a limited resource in several countries, including the Mediterranean and Middle East countries. Renewable water resources have seen a reduction of up to about 60% in the last 10 years. Under this state of affairs, the already scarce sources for untreated water sources of appropriate quality for RO or NF treatment are becoming almost unattainable, and raw waters of lower quality have to be considered as entrant for treatment, including membrane softening and desalination.
A number of governments have issued large-scale programs to recover and reuse treated municipal wastewaters, restore saline and contaminated wells and other sources, and desalinate brackish and marginal water sources. Some activity has been undertaken regarding agricultural industries by private companies to desalinate sea and other high-salinity raw waters for multiple applications.
In the field of membrane filtration a differentiation is operative among diverse varieties of membrane processes on the basis of the size and geometry of the particles to be retained. RO is capable of producing very clean water and high concentrate/retentate; however, the process can be very expensive, due to the relatively sophisticated technology it utilizes. UF, on the other hand, is relatively inexpensive but is sometimes not effective enough to meet rigorous recycling standards. Hence, a search for good UF membranes is at the front of the technology nowadays. Moreover, the process of NF can be also an effective compromise between RO and UF for some water sources. NF is easier to implement and less expensive than reverse osmosis. Because NF uses less fine membranes, the feed pressure of the NF system utilizes a working RO of seawater.
Polymeric membranes may be isotropic or asymmetric (anisotropic) in their pore structure. Isotropic membranes have a uniform pore structure throughout the membrane in contrast to the asymmetric membranes. Membranes may also be non-porous as in RO membranes.
RO membranes are obtained from casting either a polymer melt or a solution whereas asymmetric polymeric membranes are usually produced by phase inversion method. In these techniques, homogeneous polymer solution consisting of polymer and solvent becomes thermodynamically unstable due to different external effects and phase separation occurs. The formation of asymmetric membrane structure is controlled by both the thermodynamics of casting solution and the kinetics of transport process.
Membrane formation occurs by egress of solvent and ingress of non-solvent into the cast solution, leaving a two-phase system. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvent and nonsolvents, fills the pores. Depending on the evaporation/quenching conditions, initial thickness and composition of the polymer solution, different membrane structures can be obtained.
Asymmetric membranes are characterized by a very thin and dense skin layer supported by a more open porous sublayer. The dense skin layer determines the separation performance while the porous sublayer provides mechanical support and influences the overall flow resistance.
Membrane structure, especially pore size and its distribution, can be controlled for each specific application depending on the choice of the polymer, solvent, nonsolvent and preparation conditions.
RO membranes can be symmetric or asymmetric depending how they are casted, however most of the NF or UF membranes are asymmetric, with different pore sizes among the membrane. The pore-size of the membrane on the side of the solute are smaller than those on the permeate side, thus avoiding the blockage of the membranes. The stability of the membranes and the pore size under various conditions is extremely important, because it determines the lifetime of the membrane and the number of potential applications using this technology. A quantitative criterion for the retention characteristics of a membrane is the molecular weight cut off, which is defined as the molecular weight at which 90% of the solutes are retained by the membrane. Additionally, pore-size distribution (for NF and UF), charge effects, hydro- and/or lipo-philicity, and polarity of the medium will influence the truly permeability of the membrane. Furthermore, for solute macromolecules, the molecular shape in solution of the molecules plays an important role. For example, folding molecules are more efficiently retained by membranes, as compared to linear elongated molecules, of similar molecular weight. To describe the physical processes that take place in membrane filtration processes, other parameters like pressure, dielectric parameters, permeability of the membrane, have to be taken into account.
Therefore, NF and UF are essentially a lower-pressure version of RO where the purity of product water is not as critical as high grade water, or the level of dissolved solids to be removed is less than what is typically encountered in brackish water or seawater or to an application where the high salt rejection of RO is not necessary. NF is capable of removing hardness elements such as calcium or magnesium salts and also capable of removing bacteria and viruses as well as organic-related colour without generating undesirable chlorinated hydrocarbons and trihalomethanes (ozone risk compounds—only if they are volatile—they are more dangerous as carcinogenic). Nanofiltration is also used to remove pesticides and other organic contaminants from surface and ground waters to help ensure the safety of public drinking water supplies.
The processes of RO and NF are affected by the charge of the particles being rejected. Thus, particles with larger charges are more likely to be rejected than others non-charged particles; therefore, the dielectric properties of the membrane are an important subject to increase rejection.
The dielectric exclusion, which is caused by the interactions of ions with the bound electric charges induced by ions at interfaces between media of different dielectric constants, is considered as one of mechanisms of filtration. In addition, the dielectric exclusion from pores with closed geometry like circular cylinders is essentially stronger than that from pores with relatively open geometry like slits.
Besides the casting from melts or from solutions to obtain RO membranes, asymmetric membranes for nanofiltration are mostly fabricated by a process called phase inversion, which can be achieved through three principal methods: immersion precipitation (wet-casting), dry-casting and thermally-induced phase separation.
The methods known in the art do not permit to control membrane pore size and the pore size distribution. Therefore, several efforts have been applied to develop new methods for homogeneous nanopore creation.
Aromatic polysulfones (PSU) of the structure 1 below are a family of high-performance engineering thermoplastics that contain sulfone groups attaching phenoxide rings in the backbone skeleton. They are obtained by reaction between bisphenol A and di-p-dichlorodiphenylsulfone:

Since their development in the 1960's, polysulfones have been used extensively as membrane materials, mainly in the field of UF and RO, but other industrial and medical applications are also well known. These polymers display excellent oxidative, thermal, and hydrolytic stability with excellent strength and flexibility, good mechanical and film-forming properties, and resistance to extremes of pH, oxidation and acid catalyzed hydrolysis. Despite these benefits, however, they have some disadvantages. Their rather hydrophobic nature is of considerable limitation in some aqueous membrane applications that demand hydrophilic character. An enhancement in hydrophilicity has been achieved by different physical and chemical surface treatment procedures on preformed polysulfone membranes or by doping the casting solution of the membranes with several additives, such as other hydrophilic polymers, e.g., polyvinylpyrrolidone (PVP), to reduce fouling and to confer additional desirable properties to the membrane. Another different method for changing the surface properties of a synthetic membrane is the chemical modification of the polymer (adding functional groups via substitution) prior to the casting thus allowing the formation of new membranes from the modified derivatives. The chemical modification affords the possibility of introducing ion-exchange groups onto the polymer backbone, potential cross-linking sites and attachment sites for complexation of hazardous or specific contaminants existing in water (U.S. Pat. No. 3,709,841).
For these purposes, a variety of functional groups have been introduced onto the polysulfone polymers. Carboxylation and sulfonation procedures have led to hydrophilic and cation exchange membranes (Noshay and Robeson, 1976). Halomethylation reactions (chloro- and bromomethylation) have led to useful intermediates for anion exchange and other functionalized derivatives.
Lithiation is also a versatile polysulfone modification tool for functionalization of polysulfones. From the lithiated intermediates, among others, carboxylated polysulfones can be obtained on addition of CO2, which is a useful membrane material in UF, NF and RO processes because of its enhanced hydrophilicity (Tremblay et al., 1991; U.S. Pat. No. 4,894,159).
Polysulfones with N-containing functional groups have also been prepared via lithiation reactions (Rodemann and Staude, 1994; Rodemann and Staude, 1995).
Lithiation of polysulfones is a heteroatom-facilitated process: the sulfone group directs the lithium to the adjacent ortho positions as shown in the formula below due to strong electron-withdrawing effect induced by the sulfone group (Guiver et al., 1988; Guiver et al., 1989).

Sulfonated aromatic polysulfones synthesized by attaching sulfonic acid groups in polymer modification reactions (post-sulfonation route) have been investigated since the pioneering work of Noshay and Robeson, who developed a mild sulfonation procedure for the commercially available polysulfone (Noshay and Robeson, 1976). This approach found significant interest in the area of desalination membranes for reverse osmosis and related water purification applications (Johnson et al., 1984). Different sulfonation agents have been employed for this modification such as chlorosulfonic acid and a sulfur trioxide-triethyl phosphate complex. In these post-sulfonation reactions, the sulfonic acid group is restricted to the activated position ortho to the aromatic ether bond (through aromatic electrophilic substitution) as indicated in the formula below:

Incorporation of phosphorus groups onto polymer backbone can afford excellent thermal stability and flame retardancy. The phosphorus brings its flame-retardation effect through the formation of high char yield. While under heat, the phosphorus-containing groups first decompose and then form a phosphorus-rich residue. This residue helps to prevent further decomposition of the polymer through heat resistance and raises the decomposition temperature of the polymer to higher level.
There is considerable evidence of the interest in the Lewis acid binding properties of organoboron compounds for organic synthesis and molecular recognition. Trigonal Sp2 hybridized boronic acids RB(OH)2 bind hydrophilic diols, either by the reversible formation of a neutral trigonal boronate ester, or through a mechanism which is considered to be more favorable involving an ion-paired Sp3 hybridized tetrahedral anion. The incorporation of electron-deficient boron centers into polymer structures is particularly intriguing as it, for example, provides an opportunity to further manipulate the polymers via donor acceptor bonding. Boron containing polymers also play a major role as intermediates in the synthesis of functionalized polymers with polar side-groups and are used as polymeric electrolytes for batteries, sophisticated flame retardants, and as preceramic and photoluminescent materials.
The national demand for better environmental solutions and cleaner technologies has brought the membrane technology into the scientific forefront. Therefore there is an imperative quest for better and simple reverse osmosis, nanofiltration and ultrafiltration membranes for a high flux and high retention of solutes without the clogging of the membrane that are able to work under highly drastic mechanical and deteriorating chemical conditions. Moreover a quest for any specific membranes that is able to perform much better than those today will allow an even better use of membrane-price-technology-application result.